In nonmucosal tissues (e.g., spleen, liver, kidney), the body’s primary defense is the classical proinflammatory adaptive, antigen-specific immune response orchestrated in the local draining lymph nodes with afferent and efferent lymph flow. The major adaptive immune cells in the systemic immune system are T lymphocytes with αβ T cell receptors (TCRαβ T cells) for specific antigen recognition and plasma B cells that synthesize immunoglobulin M (IgM) and immunoglobulin G (IgG) complement–binding antibodies, which can induce inflammation. However, mucosal tissues (i.e., those of the respiratory, gastrointestinal, and urinary systems, as well as the eyes, nose, throat, and mouth) must constantly discriminate between what is harmful and what is not, and although inflammation is protective, it usually disrupts the normal physiologic processes and is not desirable unless absolutely necessary. Accordingly, mucosal tissues have developed specialized “noninflammatory” defense mechanisms, which form the basis of the mucosal immune system and can function independently of the systemic immune system. The mucosal immune system contains both specialized innate lymphoid cells (macrophages, natural killer cells, dendritic cells [DCs]) and adaptive T lymphocytes with γδ T cell receptors (TCRγδ T cells) and plasma B cells that synthesize immunoglobulin A (IgA), a nonclassical complement-binding antibody. These innate and unique adaptive responses can prevent or limit responses to foreign nonpathological agents while eliminating pathological agents/substances with little or no inflammation. In addition, if this front-line defense system fails or is bypassed, the lungs do have a classical adaptive immune response system in which lymphatic drainage is via the mediastinal lymph node located in the upper region of the thoracic cavity adjacent to the main left-right lung bifurcation.
A distinctive feature of the mucosal immune system is that antigens are processed through lymphoid aggregates rather than through a true lymph node. Unlike a true lymph node, which has afferent and efferent lymph flow, lymphoid aggregates have only afferent drainage of material into the aggregate without efferent flow. This lymphoid network is commonly referred to as mucosa-associated lymphoid tissue (MALT); in the gastrointestinal tract, it is referred to as gut-associated lymphoid tissue (GALT); and in the lungs, it is known as bronchus-associated lymphoid tissue (BALT;
Fig. 26.2 ). BALT is present in the conducting airways, where the epithelium is composed mainly of ciliated cells with consistent mucus flow. However, an interesting feature of BALT is that its airway epithelium is not ciliated; it is referred to as a lymphoepithelium, which creates a break in the mucus flow (like a drain) and allows the substances/ particulates to be processed in the lymphoid aggregate (or follicle).
It appears that there is communication between mucosal tissues and that sensitization via one organ is transposed to all MALT/BALT tissues via a lymphatic-like drainage network. The systemic immune system and MALT/BALT may work independently of each other, and the fact that one is sensitized may not be true of the other. This may serve as a defense mechanism in limiting sensitization only to mucosal tissue. Lymphocytes also cluster in smaller numbers and density in what is referred to as tertiary ectopic lymphoid tissue (TELT), in which they can also process antigens (see